Printed circuit board assembly

ABSTRACT

A PCB assembly ( 1 ) in this case a DC-DC converter comprising a single layer board ( 2 ), mounts power semi-conductor devices forming high heat generating components ( 3 ) and various cores of magnetic material forming heat dissipating components ( 4 ). Tracks of heat conductive coupling material ( 6 ) lie above or below each heat generating component ( 3 ) and project into one of the heat dissipating components ( 4 ) and beside the others. In one embodiment, the heat generating components ( 3 ) are housed within a heat dissipating component ( 3 ). In another PCB assembly, there is an additional plug-in PCB which may itself carry heat generating components ( 3 ) or only heat dissipating components ( 4 ). In the latter case, the heat generating components ( 3 ) are mounted on the PCB assembly below the additional plug-in PCB.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/IE00/00106 which has an Internationalfiling date of Sep. 13, 2000, which designated the United States ofAmerica.

INTRODUCTION

The present invention relates to a printed circuit board (PCB) assemblyof the type comprising a plurality of components having differentthermal attributes, namely, of different relative heat generating andheat dissipating properties over the operating range of the PCB.Further, the invention is particularly directed to the provision of PCBsfor power conversion use, whether they be for DC to DC or AC to DC powerconverters.

The majority of power conversion products manufactured today usethrough-hole mounted components on a PCB with thermal management of themain power dissipating elements achieved either using small heatsinksfor individual devices or groups of devices, or using athermal-conductive mechanical assembly to couple such heat dissipatingelements to an external heatsink. This construction technique is notcompatible with modem automated manufacturing techniques and is notefficient in the context of volumetric efficiency, with a relativelylarge volume occupied by power dissipating elements and theirheatsinking arrangements.

In more recent implementations, as available in many commercial DC-DCmedium power converters (up to 100 W typically), the windings areintegrated into a single multilayer printed circuit board along with thepower devices. Such DC-DC converters using integrated planar magneticsare manufactured by Philips or by Synqor Inc. which latter company sellssuch a converter under the Trade Mark PowerQor™. Typical examples of theterminal coupling for such multilayer printed circuit boards to metallicstructures are described in U.S. Pat. Nos. 5,973,923 (Jitaru) and5,990,776 (Jitaru). This single board construction technique is verypractical for such medium-power DC-DC converters, and one element of theinvention relates to techniques for improving heat management withinsuch modules and in their mounting arrangements These modules up torecently favoured the use of enclosed constructions with the mainheat-dissipation elements closely thermally coupled to a base plate, onwhich a heatsink could be mounted. In many cases, potting in athermally-conductive material can be used, but this approach is costly,may raise environmental issues, prevents rework, and can cause stress oncomponents even if a barrier layer is used. Recent practice has begun tofavour the use of the single-board open frame construction, facilitatedby use of semiconductor devices which can give high operatingefficiency, and operating without an external heatsink. This practicemakes thermal management within the module and to its environment morecritical.

The single-board construction approach requires, however, an excessivearea in the case of medium-power converters, particularly in the case ofAC-DC converters where the minimum height is effectively determined byitems such as electrolytic capacitors which must store energy during thelow-voltage parts of the incoming AC waveform. As a result, suchconverters using the single-board multilayer approach will have poorvolumetric efficiency, unless of course several lower power submodulesare stacked in order to fit within the height constraints imposed by theelectrolytic capacitor or a similar bulky element. This approach,however, adds to cost, as switching stages need to be replicated in eachmodule and there is a cost associated with mounting and connecting thesub-modules, as well as the thermal management issues associated with astack of converter sub-modules as part of an overall power conversionmodule.

The heat generation of any particular component in, for example, a PCBforming part of a power converter, will vary depending on the operatingconditions of the power converter module. Typically, components in whichconduction losses dominate will generate more heat at lower inputvoltage within the specified range, while components in which switchingloss or magnetic core loss dominates may generate higher losses athigher input voltages. Thus, the term “heat generating” or “heatdissipating” when referring to the thermal attributes, capacities orproperties of a particular component and similarly the qualificationshigh and low of such terms, refers not to the absolute heat generatingor heat dissipating property or ability but simply to its property inthat actual specific situation. The heat dissipation property of acomponent depends largely on its inherent physical make-up. Thus, largebulky metallic components with exposed surfaces will dissipate more heatthan those smaller compact components low conductivity materials.

As the designs become more efficient, the operating temperature underwhich the components operate becomes more critical. While many of theapproaches discussed above and many of the techniques such as, forexample, the use of heatsinks such as described in U.S. Pat. No.5,075,821 (Donnel), appreciate the need to dissipate the heat from someof the components with high heat generating capacity, not enoughattention has been paid heretofore to the need to operate the magneticelements, whether they be conventional magnetic elements or planarmagnetic elements at the optimum temperatures. Indeed, many of theferrite materials used-in magnetic elements are often optimised foroperation at approximately 100° C. and thus, under typical ambienttemperature and airflow conditions, the magnetic elements are notoperating at the ideal temperature. Many of the power conversion modulesof the prior art may cool the semi-conductor power componentssufficiently but unfortunately do not operate with the magneticcomponents at the optimum temperature.

The present invention is directed towards overcoming these and otherproblems with the prior art and in particular to providing an improvedconstruction of PCB and in particular an improved construction of PCBfor use for power converter elements and also to the provision of anefficient magnetic element for use with such PCBs.

STATEMENTS OF INVENTION

According to the invention, there is provided a PCB assembly of the typecomprising a plurality of components having different thermalattributes, namely, of different relative heating generating and heatdissipating properties over the operating range of the PCB wherein atleast one high heat generating component is thermally linked to a highheat dissipating component. In this way, there is an active managementof the thermal properties or generation of the PCB which can beparticularly effective in power conversion units. The PCB no longerrelies on, for example, heatsinks or the like which may be used todissipate the heat from high heat generating components but utilises theheat dissipating properties of the high heat dissipating components.

Preferably, the components are thermally linked by a heat conductivecoupling material which may, for example, be in direct contact with oneor both of the components. Such heat conductive coupling material can behoused within at least one of the components. It will be appreciatedthat the advantage of this is that further heat dissipation will beachieved.

The heat conductive coupling material may form additional tracks on theboard or additional pads and may form thermal vias with one component onone side of the board and the other components on the opposite side.These would be additional heat conductive tracks or pads, for example,of copper, over and above those used for the conduction of electricalsignals. Additionally, a conformable thermally conductive materialespecially an electrically insulating one can be particularly usefulwith some components, particularly with non-planar surfaces. Indeed,electrical conductors could be made larger than necessary in certainsituations to utilise the heat dissipating properties of them.

Ideally, the components are in close physical proximity with minimal airbetween them and in one embodiment of the invention, the heat generatingcomponent is housed at least partially within the heat dissipatingcomponent. Alternatively, the heat dissipating component can be mountedabove the heat generating component which heat dissipating component canbe a magnetic component. It will be appreciated that the great advantagefor the magnetic component is that it is now receiving heat and beingheated to allow the ferrite approach optimum thermal operatingconditions.

In one embodiment of the invention, the magnetic element is a separatemagnetic surface mount PCB carrying plug-in legs for mounting on thePCB. This surface mount PCB may be a multilayer circuit board. Thislatter surface mount PCB may form part of a power converter comprisingpower semi-conductors on the PCB below the surface mount PCB.

In this latter embodiment, preferably a layer of conformable thermallyconductive material fills the space between the bottom of the surfacemount PCB and the power semi-conductors.

It will be appreciated that ideally the heat dissipating component isthermally linked to more than one heat generating component or indeedmore than one heat generating component is thermally linked to more thanone heat dissipating component. When the heat generating and dissipatingcomponents have different thermal attributes over the PCB operatingrange, the choice of components for thermal linking is chosen to provideoptimum heat transfer over the PCB operating range. Thus, it is possiblethat the two different heat generating components would be connected tothe one heat dissipating component which heat dissipating componentwould not, in fact, over the total range of the operation of the PCB,experience any great fluctuation in the amount of heat transmittedthereto for subsequent dissipation.

Further, the invention provides a magnetic element for use with a basePCB comprising a separate magnetics element surface mount PCB carryingplug-in legs for mounting on the base PCB.

In this latter element, the surface mount PCB is a multilayer circuitboard.

Further, the invention provides a power converter comprising one ofthese latter magnetic elements and power semi-conductor elements on thebase PCB which preferably are so arranged that the surface mount PCB isabove the power semi-conductors.

In this latter power converter, ideally a layer of conformable thermallyconductive material fills the space between the bottom of the surfacemount PCB and the power semi-conductors.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a plan view of a DC-DC power converter of single PCB boardconstruction.

FIG. 2 is a cross sectional view in the direction of the arrows II—II ofFIG. 1,

FIG. 3 is a plan view of a heat coupler according to the invention in anE-core;

FIG. 4 is a side view of the heat coupler of FIG. 3,

FIG. 5 is a sectional view of a magnetic element surface mount PCBaccording to the invention mounted on a base PCB,

FIG. 6 is a plan view of a partially assembled PCB with powersemi-conductor components mounted thereon,

FIG. 7 is a sectional view of the PCB of FIG. 6 showing in section amagnetic component mounted thereon,

FIG. 8 shows another construction of surface mount PCB according to theinvention,

FIG. 9 shows a magnetic component according to the invention, and

FIG. 10 illustrates another construction of surface mount PCB accordingto the invention.

Referring to the drawings and initially to FIGS. 1 and 2, there isprovided a DC-DC power converter module in the form of a PCB assembly 1comprising a single layer 2 mounting power semi-conductor elementsforming high heat generating components 3 and various cores of magneticmaterial forming heat dissipating components 4. The heat generatingcomponents 3 are thermally linked to the heat dissipating components 4by tracks of a heat conductive coupling material 6. In this embodiment,the tracks 6 actually project into one of the heat dissipatingcomponents 4 and lie above or below each of the heat generatingcomponents 3 but are electrically insulated therefrom. Suitableinsulating materials are used.

Referring to FIGS. 3 and 4, there is illustrated a heat coupler,indicated generally by the reference numeral 10, having a base portion11 and tines 12. The base portion 11 is cranked so as to be coupledthermally closely to a suitable heat generating component by overlyingit, but not necessarily touching it, or, if touching it, being insulatedtherefrom. The tines 12 would then be allowed, for example, to projectinto the core of a magnetic device, for example, an E-shape core, shownin section and identified by the reference numeral 13. The heat coupler10 may be a metallic stamping, for example, copper strip, so as to allowthe heat to transfer laterally from the heat generating component 3 tothe heat dissipating components 4.

Referring now to FIG. 5, parts similar to those described with referenceto the previous drawings are identified by the same reference numerals.In this embodiment, the PCB board 2 has mounted on it by conventionalboard Interconnect legs 15, a heat dissipating component formed from aseparate magnetic surface mount PCB 16 mounting planar ferrite magneticcores 17. A thermal interface sheet 18 is interposed between themagnetic core 17 and the PCB 2. Thermal vias 19 interconnect the thermalinterface sheet 18 with the heat generating components 3, in this case,power semiconductor elements. The thermal vias 19 will be filled with asuitable heat conductive coupling material and similarly so will thethermal interface sheet 18 be manufactured from such a material. Thesurface mount PCB 16 is a multilayer printed circuit board.

Referring now to FIGS. 6 and 7, parts similar to those described withreference to the previous drawings are identified by the same referencenumerals. In this embodiment, there is illustrated a heat dissipatingcomponent formed from a magnetic core 20 within which are housed heatgenerating components 3. A thermal connector 21 is also provided toensure that the high heat generating components 3 are thermally linkedto the high heat dissipating components, namely, the magnetic core 20.

Referring now to FIG. 8, parts similar to those described with referenceto the previous drawings are identified by the same reference numerals.In this embodiment, the surface mount PCB 16 has mounted thereon aplurality of heat generating components 3 and a heat dissipatingcomponent formed from the magnetic core 17. Thermal connectors formedagain from tracks of thermally conductive coupling material 6 areprovided. This surface mount PCB 16 could effectively be a whole powerconverter which can then be readily easily mounted on the PCB assembly 1which effectively forms a base PCB and removed therefrom whenmaintenance is required.

Referring now to FIG. 9, it will be appreciated that obviously heattransfer from one face of a core of magnetic material to the other faceof the core magnetic material may be achieved by close alignment ofmating surfaces of the magnetic material and the use of appropriateadhesives. As will be appreciated, gaps may be required in the case ofinductors and transformers carrying a DC bias current.

FIG. 9 illustrates such a core of magnetic material 30 in which a gap isfilled with a heat conductive coupling material, almost certainly aconformable material 31. In this embodiment, the surface mount PCB 16 isa multilayer circuit board which, it will be seen, is mounted above thepower generating components 3.

FIG. 10 illustrates an alternative PCB assembly indicated generally bythe reference numeral 40, in which parts similar to those described withreference to the previous drawings are identified by the same referencenumerals. In this embodiment, the surface mount PCB 16 again mounts theplanar ferrite magnetic core now directly over the heat generatingcomponents 3 or the PCB 2.

It will be appreciated that the heat generating and heat dissipatingcomponents can be linked without necessarily touching. Simply placingthem together or one within the other, as illustrated in the drawings,will be sufficient to have good heat conductive coupling. Obviously, theuse of any form of heat conductive coupling material is advantageous andin many instances, with uneven and irregular surfaces and components, aconformable heat conductive material will be particularly useful. Oneparticular form of conformable thermally conductive but electricallyinsulating material is that sold under the trade mark GapPad byBergquist Corporation. Close alignment of components and the correctchoosing of components is all important.

Current ferrite materials have a thermal conductivity of the order of 5Wm-1 K-1.

In a typical 100 W converter module using two E22 corers with a totalface in contact with the printed circuit board on one side of 320 mm2and a material thickness of 2.5 mm, the thermal impedance is about 2 K/Wper face. With a typical dissipation of about 10 W, the opportunity forachieving very effective cooling of the relatively small powersemiconductor devices by using the ferrite material as a heattransmission medium to an external heat dissipation surface is evident.

Various other forms of thermal conductivity can be used such as baseplates, heatsinks, etc. as shown in the prior art, however, they do notform any essential elements to the present invention. The arrangementaccording to the present invention, allows the magnetic elements to bethe main heat transfer devices from the power semi-conductor elements.

In the case of a large class of converter modules, typically those withAC input, the practical height is determined by energy storage elementssuch as electrolytic capacitors. There is a corresponding restriction onthe area or “footprint” which can be taken up by the power conversionmodule. In this case, it may no longer be advantageous on grounds ofcost or volumetric efficiency to use a construction technique where themagnetic elements are integrated within a single multilayer boardconstruction.

In this case, it is advantageous to have a base printed circuit boardsuch as illustrated in FIG. 10 in which the power semi-conductor devicesare mounted below the magnetic core and use thermal vias to spread theheat within the board and/or to conduct heat to the lower face of theboard. The layer count in this board can be two or four, considerablycheaper than the higher layer counts typically used in the case ofplanar magnetic in-board winding implementations. A low-profileimplementation of the magnetic elements, with windings implemented asprinted circuit boards or in another low-profile implementation andpassing through the window area, may then be mounted over the baseprinted circuit board in the module. To assist rework and test, it isadvantageous to make the magnetic assembly easily removable using a plugand socket arrangement 41 such as illustrated in FIG. 10. In the case ofsmaller magnetic assemblies, the use of connectors on the base printedcircuit board of the module and the printed circuit board in themagnetic assembly with some retention arrangement may provide asatisfactory mechanical fixing. In the case of larger magneticstructures, such connector arrangements may be augmented by conventionalfasteners.

When the low-profile magnetic structure is mounted over the base printedcircuit board, there are several options in relation to its placementrelative to the components below. Close thermal coupling may be achievedbetween components located immediately below the magnetic materialtypically employed, with appropriate layers of shielding and/orelectrical insulation (typically thermally conductive conformable orcompressible material) as required.

As an alternative, the magnetic element may be located flush with thebase printed circuit board, or over low-height components, and the powersemi-conductor elements may be mounted closer to the connectors fillingthe void that typically exists where the windings protrude beyond thecore in most planar magnetic implementations. Opportunities for upwardthermal transfer in this case can include use of the connectors andcabling in order to achieve material power dissipation, along with useof heat spreaders to achieve thermal coupling to the magnetic material.

Measures can be taken as above to improve the face-to-face thermalconductivity of the magnetic material, including careful thermalmanagement at interfaces, as outlined above.

Given an E64 core set in El configuration, and assuming the transmissionis only through the ferrite (i.e. no transmission through the windingwindow), a face-to-face thermal resistance assuming a ferrite thermalconductivity of 4 Wm-1 K-1 is calculated as 3.7 K/W. This figure can beincreased by greater thermal “filing” of the magnetic window and asmagnetic matenals improve, but is a figure which may achieve asatisfactory cooling effect in the case of many circuit configurations.

The invention provides a relatively simple way of improving the thermalperformance of such PCBs by ensuring that the high heat generatingcomponents are thermally linked to the high heat dissipating components,whether they be directly coupled by a heat conductive coupling materialor simply placed very close to each other. In certain cases, there mayeven be contact. Additional heat conductive tracks, pads, thermal vias,etc. may all be used. The invention does not envisage limiting in anyway the number of layers or tracks making up the PCB.

Also, it will be appreciated that the provision, according to theprevent invention, of a separate magnetic surface mount PCB carryingplug-in legs for mounting on the base PCB is particularly advantageous.A multilayered circuit board is often used to provide planar magneticswhich, if they need to be replaced, can only be replaced withdifficulty. Both heat dissipating components and heat generatingcomponents will be connected and linked to more than one or othercomponents. It will also be appreciated that it will be necessary toensure that generating and dissipating components having differentthermal attributes over the PCB operating range, are chosen such as toensure that optimum heat management performance is achieved over thefull PCB operating range.

It will be appreciated that power converters manufactured in accordancewith the invention will be particularly advantageous in use.

In the specification the terms “comprise, comprises, comprised andcomprising” or any variation thereof and the terms “include, includes,included and including” or any variation thereof are considered to betotally interchangeable and they should all be afforded the widestpossible interpretation.

The invention is not limited to the embodiments hereinbefore describedbut may be varied within the scope of the claims.

1. A printed circuit board (PCB) assembly comprising: a plurality ofcomponents in said PCB assembly having two sets of different thermalproperties over an operating range of said PCB assembly; a firstcomponent set having heat generating properties; a second component sethaving heat dissipating and magnetic properties; means for thermallylinking said first component set to said second component set wherebyheat generated by at least one component from said first component setis dissipated by at least one component from said second component set.2. The PCB as claimed in claim 1 wherein the thermally linking meanscomprises a heat conductive coupling material to couple at least onecomponent from said first component set to at least one component fromsaid second component set.
 3. The PCB as claimed in claim 2, in whichsaid heat conductive coupling material is in direct contact with one ofsaid components.
 4. The PCB as claimed in claim 2, in which said heatconductive coupling material is housed within at least one of saidcomponents.
 5. The PCB as claimed in claim 2, in which said heatconductive coupling material forms tracks on said board.
 6. The PCB asclaimed in claim 2, in which said heat conductive coupling materialforms pads on said board.
 7. The PCB as claimed in claim 2, in whichsaid heat conductive coupling material forms thermal vias with onecomponent on one side of said board and said other component on theopposite side.
 8. The PCB as claimed in claim 2, in which said heatconductive coupling material is a conformable thermally conductivematerial.
 9. The PCB as claimed in claim 1, in which said components arein close physical proximity with minimal air gap between them.
 10. ThePCB as claimed in claim 1, in which said heat-generating component ishoused at least partially within said heat-dissipating component. 11.The PCB as claimed in claim 1, in which said heat dissipating componentis mounted above the heat generating component.
 12. The PCB as claimedin claim 1, in which a magnetic component from said second component setis a separate magnetic surface mount PCB carrying plug-in interconnectlegs for mounting on said board.
 13. The PCB as claimed in claim 12, inwhich said surface mount PCB is a multilayer circuit board.
 14. The PCBas claimed in claim 12, in which said surface mount PCB forms part of apower converter comprising power semi-conductors on said board belowsaid surface mount PCB and in which a layer of conformable thermallyconductive material fills the space between the bottom of said surfacemount PCB and said power semi-conductors.
 15. The PCB as claimed inclaim 1, in which said heat-dissipating component is thermally linked tomore than one heat-generating component.
 16. The PCB as claimed in claim1, in which said heat-generating component is thermally linked to morethan one heat-dissipating component.
 17. The PCB as claimed in claim 1,in which when said heat generating and said dissipating components havedifferent thermal attributes over said PCB operating range, the choiceof components for thermal linking is chosen to provide optimum heattransfer over said PCB operating range.
 18. A printed circuit board(PCB) assembly comprising: a plurality of components in said PCBassembly having two sets of different thermal properties over anoperating range of said PCB assembly; a first component set having heatgenerating properties; a second component set having heat dissipatingand/or magnetic properties, at least one component comprising a magneticelement having a separate magnetic element surface mount PCB carryingplug-in legs for mounting on said board of said PCB assembly; means forthermally linking said first component set to said second component setwhereby heat generated by at least one component from said firstcomponent set is dissipated by at least one component from said secondcomponent set.
 19. The PCB as claimed in claim 18 wherein the thermallylinking means comprises a heat conductive coupling material to couple atleast one component from said first component set to at least onecomponent from said second component set.
 20. The PCB as claimed inclaim 18, in which said magnetic element surface mount PCB is amultilayer circuit board.
 21. A power converter having a printed circuitboard (PCB) assembly comprising: a plurality of power semi-conductorcomponents in said PCB assembly having two sets of different thermalproperties over an operating range of several PCB assembly; a firstcomponent set having heat generating properties; a second component sethaving heat dissipating and/or magnetic properties, at least onecomponent comprising a magnetic element having a separate magneticelement surface mount PCT carrying plug-in legs for mounting on saidboard of said PCB assembly; means for thermally linking said firstcomponent set to said second component set whereby heat generated by atleast one component from said first component set is dissipated by atleast one component from said second component set.
 22. The powerconverter as claimed in claim 21 wherein the thermally linking meanscomprises a heat conductive coupling material to couple at least onecomponent from said first component set to at least one component fromsaid second component set.
 23. The power converter as claimed in claim21, in which said surface mount PCB is arranged above saidsemi-conductor elements.
 24. The power converter as claimed in claim 21,in which a layer of conformable thermally conductive material fills thespace between the bottom of said surface mount PCB and said powersemi-conductors.